Page:Popular Science Monthly Volume 41.djvu/275

Rh Roberts of this nebula shows a planetary system at a somewhat advanced stage of evolution; already several planets have been thrown off, and the central gaseous mass has condensed to a moderate size as compared with the dimensions it must have possessed before any planets had been formed." In 1891, after more definitely describing the appearance of Mr. Roberts's photograph, he said that "to liken this object more directly to any particular stage in the formation of the solar system would be f to compare great things with small,' and might be indeed to introduce a false analogy; but, on the other hand, we should err through an excess of caution if we did not accept the remarkable features brought to light by this photograph as a presumptive indication of a progress of events in cosmical history following broadly upon the lines of Laplace's theory."

Dr. Huggins's spectrum observations on comets, in connection with those of other observers, satisfied him of the existence of different types, and that the same comet might present on one occasion one spectrum and on another the other spectrum; that they shine partly by reflected solar light and partly by their own light, the spectrum of which indicates the presence in the comet of carbon, possibly in combination with hydrogen. In the case of the Wells comet of 1882, he remarked that as Prof. A. Herschel and Dr. Von Konkoly had showed long before that the spectra of the periodic meteors are different for different swarms, it was not surprising that we now had a comet the matter of the nucleus of which under the sun's heat showed an essential chemical difference from the long series of hydrocarbon comets which had appeared since 1864. The spectrum of Coggia's comet (1874) indicated an approach to the earth of forty-six miles per second, while the real velocity of approach was only twenty-four miles per second. It was uncertain whether the whole or part of the difference in the velocity was due to the motion of the matter within the comet. It seemed probable, therefore, that the nucleus was solid, heated by the sun, and throwing out matter which formed the coma and tail; and part of this was in a gaseous form, giving the spectra of bright lines. The other portion existed probably in small incandescent particles; the polariscope showing that certainly not more than one fifth of the whole light was reflected solar light. In a paper on Photographing the Solar Corona without an Eclipse, Prof. Huggins spoke, in 1882, of problems of the highest interest in the physics of our sun connected with the varying forms of the coronal light which seemed to admit of solution only on the condition of its being possible to study the corona continuously, and to confront its changes with other visible phenomena presented by the sun. The spectroscopic method of viewing the prominences failed; experiments in